For purposes of driving micromechanical silicon actuators (MEMS actuators) electrostatic forces have been successfully deployed for several decades. Compared with drives operating on electromagnetic, piezo-electric and thermal principles, they have the advantage that the whole micro-actuator structure, including its drives, can be implemented completely in silicon. Examples of micromechanical actuators of this kind are resonantly operated micro-mirrors, acceleration sensors and rotation rate sensors. Since no other materials, which as a rule are mismatched with respect to their thermal expansion coefficients, are used, these micromechanical actuators can be encapsulated comparatively easily and with a high yield at wafer level using current wafer bonding technologies, such as anodic bonding, eutectic bonding or glass frit bonding, despite the associated high temperatures of sometimes significantly more than 300° C. (wafer level packaging). Suitable encapsulation is essential for MEMS products so as to offer protection against contamination by particles, liquids and gases, and also against mechanical over-stressing. As a result of the option of encapsulation of these systems not only at chip level but also even at wafer level low manufacturing costs are achieved at the same time as high production yields.
These advantages cannot be achieved with drives operating on the other principles. Thus electrodynamic drives, for example, require the deposition of thick metal layers so as to instantiate planar coils with the lowest possible ohmic resistance. In addition to the already significant disadvantage of the layer stresses, or layer stress gradients, which are very difficult to avoid with metal deposition and can result in severe distortion of the actuator, and the high metallic mass that the actuator must carry with it, an even greater problem arises relating to any attempt to encapsulate such an actuator using wafer bonding techniques at wafer level. In most cases the thermal mismatches of the materials involved rule out the use of a wafer level packaging method on account of the high temperatures. As a rule only non-hermetically sealed bonding techniques remain viable for purposes of protecting the microstructure. Further significant disadvantages of the electromagnetic drives ensue from the necessity of generating, in addition to moving planar coils, an external magnetic field by means of hybrid applied permanent magnets positioned as densely as possible on the micro-actuator. Such a hybrid-mounted system is more expensive and less suitable for mass production than an electrostatic actuator. The achievable minimum volume that can be achieved for an electromagnetically driven MEMS actuator is, as a rule, significantly greater than that which can be achieved for an actuator fitted with electrostatic drives.
The proposed micromechanical actuator is of particular importance for the field of optical micro-mirror actuators, but equally can also be deployed for many other types of actuator such as switches or gyroscopes. Micro-mirrors are deployed for the targeted deflection of an incident light beam or electromagnetic radiation in other wavelength ranges (IR, UV). As a rule, they take the form here of thin etched plates of silicon, which are mirror coated either dielectrically or with very thin metallic layers, and are suspended on torsion or bending strips such that they can move. When operated resonantly such micro-mirrors, up to several millimeters in size, can be deflected with sufficiently large scanning amplitudes at frequencies of many kilohertz. In recent years work has been very intensive on the development of dual-axis scanning micro-mirror systems that are designed to be deployed in compact laser projection displays. A single-colour or multi-colour laser beam is directed onto the moving mirror and is deflected by the latter in two axes, vertically and horizontally, so rapidly that the human eye perceives a continuously illuminated rectangular area on the projection surface. By synchronising the modulated laser source with the mirror movement image information can then be transferred with a high resolution.
For the deflection of the laser beam over the projection surface there are two fundamentally different scanning methods of known art, the raster scan and the Lissajous scan. In the case of the raster scanning method that is preferably deployed a rapid line movement is usually combined with a slow vertical movement. For example, in order to project a an image in SVGA resolution, that is to say 600 lines with 800 pixels per line, with a refresh rate of 60 Hz, a line frequency of at least 36 kHz is required. This applies for lines projected in one writing direction, thus e.g. from left to right. During the reset time period of the line, i.e. horizontal, scanner no information is therefore transferred. If one wishes, however, to utilise both scanning directions, in other words to project lines both from left to right and also from right to left, then the frequency requirement halves to at least 18 kHz. In order still to be able to achieve sufficiently large scanning amplitudes with what remains a very high scanning frequency, horizontal scanners are usually operated in resonance, which results in a sinusoidal velocity characteristic and thus an undesirable, non-homogeneous distribution of light intensity in the line direction. While this is almost impossible to circumvent in relation to the horizontal axis, it should at least be possible to achieve optimal homogeneity of the image presentation in the vertical direction. This is ideally achieved by means of non-resonant operation of the deflection in the vertical direction with a sawtooth-shaped scanning characteristic. In order to use the light available in an optimal manner an effort is made here to configure the rapid return to the starting point—i.e. the steeply falling sawtooth flank of the vertical scan—to be as short as possible. For compact laser projection displays with resolutions in VGA format, SVGA format or greater, optical constraints dictate that a mirror plate with a diameter in the millimeter range is required. The product of the mirror diameter and a single-sided mechanical scanning angle provides the so-called Theta-D product, which can be seen as a measure for the optical resolution. Thus for example for an SVGA resolution a Theta-D product [mm×degrees] of 9.37 is required in the horizontal direction, and 7.03 in the vertical direction. So as to achieve, on the one hand, sufficient shock robustness and lack of sensitivity to vibration for the microsystem when deployed in mobile applications, for example in a mobile phone, and to achieve, on the other hand, a sufficiently rapid reset of the mirror in sawtooth operation, preferably of less than 2 ms, the lowest natural resonance of the slow axis (vertical deflection) should not be less than 1000 Hz.
Let us assume, for example, a very compact, dual-axis, cardanically suspended micro-mirror scanner, whose mirror plate has an edge length of 1 mm. The frame surrounding this mirror and its torsion spring, also suspended such that it can move (in English usually denoted as a gimbal) inevitably has a significantly larger edge length than the mirror. Let us assume a comparatively short spring length for the torsion springs of 300 μm and an additional frame width of 200 μm. With a square contour the moving frame then has an edge length L of 2 mm. For the micro-actuator we assume further a minimum thickness D of 60 μm and a minimum natural torsional frequency of 1000 Hz. To achieve an SVGA resolution a mechanical tilting movement θ of at least +/−7° is required for the moving frame.
The natural torsional frequency Fres can be expressed as:
                              F          res                =                              1                          2              ⁢              π                                ⁢                                    k              J                                                          (        1        )            
where k represents the spring constant, and J represents the moment of inertia of the mirror about the axis of rotation.
J can be expressed as:J= 1/12ρDL4  (2)
Here ρ is the density of the mirror material (density of silicon: 2330 kg/m3).
The mechanical restoring torque Tmech for the required full deflection, here assumed to be +/−7°, can be expressed as:Tmech=kθ  (3)
In addition for the maximum deflection it is true that:|Taktuator|=|Tmech|  (4)
If in equation (3) the spring constant k is replaced by the expression (1) and the given values for frequency, density, edge length and mirror thickness are inserted, then while taking into account equation (4) the torque that must be applied by the actuator Taktuator can be expressed as:
                              T          aktuator                =                              4            ⁢                          π              2                        ⁢                          f              res              2                        ⁢            ρ            ⁢                                                  ⁢                          DL              4                        ⁢            θ                    12                                        =                              4            ⁢                                                                                π                    2                                    ⁡                                      (                                          1000                      ⁢                                                                                          ⁢                      Hz                                        )                                                  2                            ·              2330                        ⁢                                                  ⁢                                          kg                                  m                  3                                            ·              60              ·                              10                                  -                  6                                                      ⁢                                                  ⁢                          m              ·                                                (                                      0                    ,                    002                    ⁢                                                                                  ⁢                    m                                    )                                4                            ·                                                7                  ⁢                  π                                180                                              12                                        =                  8          ,                      99            ·                          10                              -                7                                              ⁢                                          ⁢          Nm                    
There are indeed micro-actuators with electromagnetic drives of known art that can generate torques of this order of magnitude. However, these have the disadvantages noted above. At this point in time micromechanical actuators of known art with non-resonant electrostatic drives are unable to achieve such torques with the required size of micro-mirror and high deflections.
For the tilting movement of an actuator unit electrostatic tilt drives that have intermeshing moving and static comb-type or finger-type electrodes are of known art. In order to generate quasi-static deflections of an appreciable amplitude with the aid of such electrostatic drives, also denoted as comb-drives, only comb-type electrodes that are offset from each other vertically have come into consideration to date. In most cases the vertical offset is generated by the use of a second silicon layer, which is electrically insulated from the plane of the first electrode that is located underneath. Static and moving electrodes are generated in different planes and thereby possess the desired vertical offset. By the application of a voltage between the static and moving electrodes the moving electrode is deflected out of the plane until the electrostatic torque and the mechanical restoring torque of the spring suspension of the mirror or frame balance one another. The maximum achievable static deflection angle is defined on the one hand by the vertical offset of the electrode planes as determined by the production process, and on the other hand by the electrode geometry, namely the lateral distance of the end of the moving comb-type electrode from the axis of rotation of the actuator unit. The greater this distance, the smaller is the maximum deflection angle that can be achieved. The greater the vertical offset of the electrode planes, the greater is this deflection angle.
In order to be able to achieve large tilt angles of more than +/−5° mechanically and also quasi-statically, i.e. in non-resonant operation, it is necessary to attach the electrodes near to the axis of rotation, i.e. the tilting axis, and not to create electrode fingers that are too long. Only in this manner is it possible to generate an effective torque over a larger range of angles. However, as a result of the short lever arm the torque that can be achieved is also very much lower than with a comparable arrangement of electrodes far from the axis with a large lever arm.
Thus Young-Chul Ko et al., “Gimbaled 2D Scanning Mirror with Vertical Combs for Laser Display”, IEEE Optical MEMS and Their Applications Conference, 2006, Pages 104 and 105, shows, for example, such a dual-axis micro-mirror scanner for laser displays with vertically offset comb-type electrodes, via which the slow axis of this so-called gimbal arrangement is driven in a non-resonant manner. Here the moving comb-type electrodes are firmly connected with the actuator unit, and extend from the point of connection with the actuator unit parallel to the outer tilting axis, i.e. parallel to the slow axis. While the mirror diameter and resonant frequency in this micro-mirror scanner fulfil the above requirements, the mechanical quasi-static tilt angle obtained of +/−4.2° does not satisfy the specifications for high resolution projection.
This is only one example of many that show that up to the present time a standard design of a dual-axis micro-mirror scanner with electrodes positioned close to the axis is not suitable for achievement of the large scanning angles of more than +/−7° required for the slow quasi-static axis, if at the same time the mirror diameter is not to fall below a minimum of 1 mm, and the resonant frequency of the slow axis is not to be significantly less than 1 kHz. The high forces necessary would only be possible in conjunction with a much greater number of comb-type electrode fingers positioned near the axis. However, new problems would arise in these circumstances, since the moving mass thereby increases significantly, the sensitivity to accelerations increases and the space requirement is then not only very large along the fast mirror axis, but also at right angles to it. In particular when deploying such a dual-axis scanner in future generations of mobile phones, which are likely to be even flatter than existing models, at least one of the two chip edge lengths that lie parallel to the mirror surface must turn out to be sufficiently small to allow the mirror chip still to be integrated.
V. Milanović, “Improved Control of the Vertical Axis Scan for MEMS Projection Displays”, Optical MEMS and Nanophotonics, 2007, Pages 89 and 90, features an arrangement of known art in which, despite the use of electrostatic comb-drives, comparatively large quasi-static tilt angles can be achieved with a sufficiently high resonant frequency and mirror size. A dual-axis mirror for laser projection is described, which, however, operates in a manner other than that of the above described micro-mirror scanner. To create the two axes the mirror does not possess a gimbal arrangement, by means of which the two axes can be deflected almost totally independently of one another. Instead a mirror plate, bonded to a small platform in a subsequent operation, is deflected with the aid of the platform, which can be tilted in two axes. In principle the axes are here more strongly coupled with one another than in a gimbal arrangement. This is undesirable for the applications that are presently being pursued, however, since accurate guidance via the tilting axes is not possible. The platform is tilted via comb-drives, which are located far from the mirror, i.e. from the platform. The torques are transferred to the platform via long rods orientated towards the centre of the chip and provided with a plurality of articulations. As a result of this construction both chip edge lengths are approximately the same size and are thus unsuitable for installation in flat items of equipment, in particular, flat mobile phones. According to measured data the proposed scanner achieves a mechanical vertical total deflection of approx. 11°, i.e. a symmetrical deflection of +/−5.5°, with a mirror plate diameter of 0.8 mm and an acceptable resonant frequency of 934 Hz. This likewise does not fulfil the above requirements for a high-resolution laser display.
On the basis of this prior art the object of the present invention consists in specifying a micromechanical actuator with a non-resonant electrostatic comb-drive that is suitable for deflecting a micro-mirror with an edge length of at least 1 mm at a natural frequency of ≧1 kHz through at least +/−7°. In a dual-axis configuration the actuator should be also be suitable for the non-resonant drive of the slow axis, and should enable simple and cost-effective production in silicon technology, a hermetic vacuum encapsulation, and also a low power consumption.